The Grateful Dead is a real-life American band. Their mascots include the dancing bears named Melody and Verse, as well as skeletons wearing colorful top hats.
After Ned Flanders unwittingly drunk a can of Groovy Grove Juice Corporation's organic juice that was laced with peyotes due to Homer's actions, he hallucinated the Dancing Bears Melody and Verse as well as skeletons from The Grateful Dead crossing the street, with the latter party tipping their top hats while bidding him good morning.
Their album "Long Strange Trip To Shelbyville" is used by the Eco-friendly Activists during Homer's beer detox, which he listened to for three days. While the album itself is fictional, its title is a reference to their real-life hit song "Truckin'" which contains the lyric "what a long strange trip it's been," and the song Homer listens to, "Shakedown Street," is actually the title track of one of the Grateful Dead's real-life albums.